Conventionally, cellulose wadding or fluff pulp is employed as the primary absorbent material in absorbent products such as babies' disposable napkins, incontinence pads for adult use and catamenials. However, although such cellulose absorbents are inexpensive, their absorbency is not especially high.
In recent years, polymers have been developed that have a very high absorbency with respect to aqueous liquids. Thus, hydrophilic polymers have been developed that can absorb more than 15 parts by weight of water per part of polymer. It can be readily envisaged that the partial or complete substitution of these so-called superabsorbent polymers for the cellulose absorbents that have been widely used hitherto might offer significant advantages by permitting the production of absorbent products that have increased absorbency and/or lower bulk. However, it has proved difficult to incorporate superabsorbent polymers into absorbent products in a satisfactory manner.
One problem with such superabsorbent polymers is that they should be prevented from coming into contact with the skin of the user of the absorbent product. Two techniques for overcoming that problem have been proposed in the art. The first technique involves the coating of one surface of a layer within the absorbent product with a hot-melt adhesive and bonding the particles of superabsorbent polymer into the product by means of that adhesive. The second technique is to confine the superabsorbent polymer particles by means of tissue paper. However, both of these techniques have the disadvantages that they involve additional expense (due to the cost of the extra material, namely the hot-melt adhesive or the tissue paper, as the case may be) and that the efficiency of the superabsorbents is impaired. Thus, whereas the hot-melt adhesive will block part of the surface area of the superabsorbent particles in the first of these prior-art proposals, the tissue paper used in the second of the proposals may provide the superabsorbent particles with insufficient space for swelling as they absorb moisture.
The prior-art proposals for incorporating superabsorbent polymers into absorbent products have generally involved the use of laminated structures. It is suggested in EP-A-0,202,472 that often the resulting products are easily delaminated with impaired absorbency. That European Patent Specification discloses a non-laminar absorbent product comprising matrix fibres (specifically cellulosic fibres or a mixture of cellulosic fibres and synthetic staple fibres) having a liquid-absorbing material (such as a superabsorbent polymer) bound within the matrix fibres by means of a heat-activated binder material. The binder may be thermoplastic or thermosetting and may, for example, be incorporated into the matrix in the form of a powder. In the exemplary embodiments of the process for producing the non-laminar absorbent product, matrix fibres are laid down in a first layer, a superabsorbent powder is evenly distributed thereover and a second layer of matrix fibres is laid down over that. Thus, in the continuous process illustrated therein, a mat is formed by laying down the matrix fibres on a web-forming device at two locations, a liquid-absorbing material being distributed amongst the matrix fibres at a location intermediate the said locations at which the matrix fibres are laid down. Such an arrangement is said to ensure that the absorbent is not exposed on one surface of the finished absorbent web (EP-A-0,202,472 at page 12, lines 18-27).